Respiratory syncytial virus is the principal cause of hospitalization for respiratory tract illness in young children, the most common cause of respiratory failure in infants and is the leading infectious cause of wheezing and asthma exacerbations in children under two years of age, extensively reviewed in Hall CB, 2001, N. Engl. J. Med., 344:1917–1928. In addition, RSV is the second most common infectious agent that causes wheezing in adults over 65 years old and can also cause severe pneumonia in this age group. For each year in the decade 1980–90, RSV infection led to the hospitalization of approximately 100,000 children for a cost of over $300 million. Currently, there is no effective vaccine for RSV and pharmacological treatment is far from optimal. Development of new therapeutic agents and vaccine approaches hold the promise of reducing morbidity and mortality from this important pathogen.
RSV is an enveloped RNA virus of the Paramyxoviridae family and has a nonsegmented, single-stranded, negative sense genome. The genome of RSV encodes 10 proteins, of which two are nonstructural. Critical to RSV-induced immunity and pathogenesis are the large envelope glycoproteins, which are the fusion protein (F) and the attachment protein (G). Variations in the G protein distinguish the two major groups of RSV strains, A and B, although there are a few differences in the F protein between the two strains.
RSV infections occur most commonly between November and May each year in the US, with a peak of frequency in January to February. Illness with the A strain is more common, but both strains cause illness during the same months of the year. Within the strains, there can be genotypic differences that more commonly cause illness within a specific community, and these strains can vary yearly, providing a possible mechanism by which previous infection may not lead to immunity. Reinfection occurs quite commonly with RSV, and lower respiratory symptoms are three-to-four more frequent with RSV reinfection compared to another respiratory virus, such as parainfluenza virus. Reinfections are not limited to children, as RSV accounts for about 21% of acute respiratory infections in the elderly attending senior day care.
Following two to eight days of incubation, RSV replicates in the epithelium of the nasopharynx and then may spread to the lower respiratory tract one-to-three days later. The pathologic features of bronchiolitis caused by RSV include epithelial necrosis and sloughing in the small airways, edema in the walls of the small airways, and increased mucus production. These three consequences of RSV infection can lead to airflow obstruction within the small airways, resulting in hyperinflation, atelectasis, and wheezing. In adults with severe pneumonia caused by RSV, radiographic findings include interstitial and alveolar infiltrates with consolidation. On histologic examination, the bronchiolitis may resolve within several days of the onset of illness; however, repopulation of ciliated epithelial cells generally does not occur for at least two weeks, and complete resolution of the cellular changes may take four to eight weeks. The histologic abnormalities correlate with the prolonged clinical course of cough, wheezing, and decreased pulmonary function.
The virus is transmitted through direct inoculation of secretions containing RSV via the hands or large particle aerosols into the eyes and nose, and very infrequently the mouth. From studies investigating the spread of RSV within the hospital, it seems that transmission of the virus necessitates close or direct contact with large droplets or fomites. RSV can also live on skin, cloth, and other objects for extended periods of time, extending the possibility of transmission of infection.
Approximately 70% of people are infected with RSV in the first year of life and almost everyone has been infected by age 3. Although the majority of people have self-limited disease, RSV is a significant respiratory tract pathogen and is a major cause of hospitalization in children. Whereas parainfluenza infection results in a hospitalization rate of 2.8 per 1,000 infections, the rate of hospitalization for RSV is roughly four-fold higher at 10 per 1,000. In hospitalized children, RSV is cultured in 50–90% of patients with bronchiolitis, 5–40% with pneumonia, and 10–30% with tracheobronchitis. In patients with cystic fibrosis, RSV accounts for 43% of hospital admissions for viral infections, more than any other virus. In adults, up to 4.4% of patients admitted to the hospital for community-acquired pneumonia have RSV identified from respiratory secretions. Chest radiographs in patients with RSV induced pneumonia may be misidentified as being of bacterial etiology since lobar consolidation can occur in up to one-third of patients.
RSV is also an important cause of wheezing and asthma exacerbations in both young children and older adults. RSV is the most frequent respiratory pathogen detected in children less than 2 years of age that present to the emergency department for wheezing. In infants who are hospitalized with RSV bronchiolitis, there is a significant association with the development of asthma and allergic sensitization up to age 7. In addition, RSV lower respiratory tract illness before 3 years of age is an independent risk factor for the subsequent development of wheezing up to 11 years of age, but not age 13, and this association is not caused by an increased risk of allergic sensitization. Admission to the hospital for RSV-induced wheezing is also not limited to children, as RSV is identified in between 5–50% of patients admitted for exacerbations of chronic obstructive lung disease, with a mortality in these patients that is almost 20%.
RSV has been identified as an important cause of death among immunocompromised patients. In patients receiving immunosuppression for transplants, RSV infection has a 30–100% mortality rate. Often, these infections are transmitted to the patient from visitors or hospital staff who are experiencing an upper respiratory tract infection and be initially thought to be a relatively inconsequential illness because of the mild symptoms at the beginning of the infection. However, RSV can lead to severe pulmonary symptoms in the transplant patient with chest radiographs showing a range of infiltrates including focal interstitial, diffuse alveolar, or dense consolidation. Diagnosis is aided by bronchoscopy with bronchoalveolar lavage, with transbronchial biopsies being performed to rule out other pathogens common in transplant patients such as cytomegalovirus.
As previously mentioned, prior infection does not lead to complete or long-lasting immunity, although prior infection does seem to protect against the development of more severe disease. Vaccines, which have provided protection against other viral illnesses such as small pox, polio, and hepatitis, have a disappointing history in protecting against RSV. In the 1960s, a formalin-inactivated vaccine trial was initiated, which unfortunately led to more severe illness with subsequent RSV infection compared to non-vaccinated subjects. Eighty percent of patients inoculated with the formalin-inactivated vaccine required hospitalization, in comparison to only 5% of controls. Two patients who were vaccinated with the formalin-inactivated preparation died and at autopsy, eosinophilic infiltrates were found in the lung. In other vaccinated patients, peripheral eosinophilia and augmented RSV-induced lymphocytic proliferative responses occurred. Patients receiving the formalin-inactivated vaccine did not have RSV-specific mucosal antibodies, and serum antibodies lacked neutralizing and fusion-inhibiting activity, which inferred that the formalin inactivation selectively modified epitopes within the G and F surface glycoproteins.
The current concept regarding protective immunity is that secretory and serum antibodies protect against upper and lower respiratory infections, while cellular responses are thought to control and terminate infection. RSV-specific serum antibodies do not predict the risk of infection or illness; however, IgG 1 antibodies against F and G have a protective effect against lower, but not upper respiratory illness in rats. Infants that have high levels of maternal antibody, or who are given exogenous antibody, have improved clinical courses compared to those who do not. Cellular-mediated immunity is presumed to be the most beneficial for recovery from illness and viral clearance. Patients who have decreased cellular immune responses have more severe and prolonged disease.
Treatment for RSV illness is currently expensive and sub-optimal. Administration of immune globulin that contains high titers of RSV neutralizing antibody prophylactically to persons at high risk for the complications of RSV disease (premature infants and those with underlying cardiopulmonary conditions) diminishes severity of illness. This requires monthly administration of RSV hyperimmune globulin or monoclonal antibody against F protein and results in decreasing the risk of subsequent hospitalization. Currently, the American Academy of Pediatrics advocates prophylaxis during the RSV season for high-risk infants without cyanotic heart disease, since patients with this condition seem to have an increased risk of adverse outcomes.
At present, aerosolized ribavirin is the only therapy approved by the FDA for the treatment of RSV infection. Ribavirin is a synthetic guanosine analogue and broad-spectrum agent that is indicated for hospitalized infants. Although ribavirin treatment is associated with improved oxygenation and clinical scores, along with a decrease in the inflammatory mediators that associated with more severe disease, ribavirin is very expensive and the beneficial effect on clinical outcome has not been proven.
Nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly used to treat the symptoms of viral upper respiratory tract infections; however, prospective, randomized, double-bind, placebo-controlled trials have not been performed to determine the efficacy of these agents in viral infections in general, nor for specific viruses in particular. The rationale for using this class of medication is that they inhibit the cyclooxygenase enzymes and decrease the production of the prostanoids, a family of arachidonic acid metabolites that mediate inflammation. There are very few animal studies which investigate the effect of NSAIDs on the outcome of viral infections, and, to our knowledge, no published reports that examine the role of this medication class on RSV. Dr. Zavagno and colleagues studied the role of NSAIDs in a murine model (BALB/c) of vaccinia virus infection. Zavagno, et al., 1987, J. Gen. Virol., 68:593–600. These investigators found that mice treated with either aspirin or indomethacin had a marked increase in mortality for vaccinia virus infection over non-treated mice. The mice treated with the NSAIDs had delayed viral clearance with inhibition of the antibody response, whereas control mice that had the higher survival rate had lower virus yield and normal antibody responses.
The effect prostacyclin has on vaccinia virus infections has been reported. Zavagno, et al., 1987, J. Gen. Virol., 68:593–600. However, for the following reasons the results obtained by use of the present invention, disclosed herein, are unexpected as compared to the information provided regarding the vaccinia virus. First, the viruses are from different families. The vaccinia virus is nonenveloped and is a large DNA virus from the Poxvirus family, while RSV is an enveloped RNA virus from the Paramyxovirus family. Viruses are classified based upon the composition of the genomes and the pathway of mRNA formation. Under this classification system, DNA viruses are found within classes I and II, while RNA viruses are present within classes III–VI. Further, the presence of a virus envelop, which consists mainly of a phospholipids bilayer and other glycoproteins, results in a significant difference in the properties of enveloped viruses as compared to nonenveloped viruses. Since RSV is so unique, the invention disclosed herein provides unexpected results. The invention disclosed herein appears to be based upon unknown properties of prostacyclin and prostacyclin analogs. Furthermore, the administration of prostacyclin in the Zavagno study was twice a day. Prostacyclin has a biologic half-life in the 1–2 minute range and therefore the dosing scheme disclosed herein has greater biologic relevance.
Across the United States, about 5,000 children die each year of RSV bronchiolitis. In addition to children, patients who receive solid organ transplants such as lungs or a heart are at an extremely high risk for RSV. Among lung transplant recipients, RSV infection carries about a 50%–75% mortality rate. The current treatment options for RSV are sub-optimal for a number of reasons. Prophylatic administration of the immune globulin that contains high titers of RSV neutralizing antibody to persons at high risk for the complications of RSV disease, such as premature infants and children with underlying cardiopulmonary conditions does diminish illness, but this therapy must be given monthly, is very expensive, and carries the risk of infection of blood products. Therefore novel treatment options need to be developed.